Excess mulch, particularly if applied right against the stem or trunk of
landscape plants, leads to constantly wet bark and conditions favorable
for
disease development.

My neighbor "Volcano Mulched" the (cringe) Bradford Pears on his side of the
driveway I share with him.
In a recent storm, one of the trees broke off right at ground level, instead
of higher up where the tree's branches divide. I attribute it to excess
moisture trapped in the volcano's peak.